The present invention relates to characterizing events in acquired digital data and more specifically a method for accurately determining the location and loss of the non-reflective events in optical time domain reflectometry data.
In telecommunications and network applications, transmitters and receivers are connected together via signal transmission cables, such as coaxial cables or optical fibers. Faults in these cables often result in undesired attenuation of signals transmitted over the cables resulting in lost information. Time domain reflectometers are used to test these cables to determine if they have irregularities, such as faults or other discontinuities, that would interfere with the transmission of information.
Optical time domain reflectometry is similar to radar. Pulses are transmitted into the medium and during the interval between pulses the return signal is examined for non-fiber events, i.e. reflective and non-reflective events. When light propagates through an optical fiber, the fiber material scatters the light in a process known as "Rayleigh scattering". Some of the light is scattered back through the fiber to the transmitter. This light is referred to as "backscatter". The backscatter signal from a pulse launched into an optical fiber decreases exponentially with distance along the fiber. A non-reflective event in the fiber, where light is lost but no light is reflected, appears in the backscatter signal as an anomalous drop over a pulse width. Locating non-reflective events is of interest when determining fiber quality and fault location. From the known index of refraction of the fiber and a plot of return signal-versus-time, the location of a non-reflective event in question can be determined relative to a known event such as the beginning of the fiber or a nearby connector or splice.
A non-reflective event has a characteristic Z-shaped pattern in the acquired signal-versus-time backscatter data. The start of the anomalous drop in the backscatter is considered the location of the non-reflective event. However, locating the start of the anomalous drop is difficult when the non-reflective event is located in noisy data. This system noise tends to obscure the true backscatter return signal. Averaging of each data point of the acquired OTDR data is required to reduce the noise in the backscatter return signal. Various methods have been used to both reduce the noise in the backscatter signal and to locate the start of the non-reflective event.
One method of determining the location of a non-reflective event is to use a two-point method. If a second data point is lower than the previous data point and the difference in the amplitude values of the data points is greater than a threshold value, then a non-reflective event is indicated. This method can be used with a waveform of collected data or in a running point by point acquisition. One drawback to this method is that extensive averaging of the data is required in order to reduce the noise as much as possible. This substantially increases the acquisition time of the OTDR and thus the time it takes to characterize the fiber and locate any faults in the fiber. Reducing the number of averages increases the likelihood that non-events will be detected as non-reflective events. To overcome this problem, the detection threshold has to be increased thus reducing the non-reflective resolution of the OTDR.
Many prior techniques for determining the location of non-reflective events have used the results of a relative rate of loss determination. The relative rate of loss determination is essentially differentiating the acquired waveform from point-to-point over a pulse width of data. Since the sample spacing for the acquired data is constant, an approximation for the differentiation is a filter that sums data values prior to the differentiation point and subtracts the sums of data points after the value. Over a region of data containing a non-reflective event, the differentiation produces a maxima representing the approximate location of the non-reflective event.
U.S. Pat. No. 5,115,439 to Holmbo et al. describes a method for detecting and characterizing anomalies in an optical fiber under test where increased averaging is used over a decreasing region containing a non-reflective event. The approximate location of the non-reflective event is detected using the previously described two-point method and a minimum number of samples for each data point. Once an approximate location is found, the region containing the event is sampled again and the relative rate of loss over the event is determined and used as the approximate location of the event. The loss over the event is also determined and compared with empirically derived loss values. The region containing the event is decreased as a function of the loss and additional samples are taken over the reduced region and added to the previous samples to reduce noise. The process of determining the relative rate of loss, reducing the region containing the event and additional averaging continues through a predetermined number of iteration. The result is a location for the event with a reduced region of uncertainty for its location.
U.S. Pat. No. 5,069,544 to Buerli describes a matched filter for determining the approximate location of events with loss. The filter function f.sub.mf is created according to the equation: EQU f.sub.mf =(p.sub.1 +p.sub.2)-(s.sub.1 +s.sub.2)
where p.sub.1 and p.sub.2 are the values of the two preceding data points and s.sub.1 and s.sub.2 are the values of the two succeeding data points. The matched filter function generates a peak whose location is the approximate location of the fault and whose baseline height is proportional to the loss of the fault.
U.S. Pat. No. 4,898,463 to Sakamoto et al. describes an optical time domain reflectometer with an automatic measuring function of optical fiber defects. A level computing section sequentially performs differentiation of the acquired waveform data to provide a predetermined number of level difference data between two points of an interval "a". The interval "a" between two points of data being subjected to differentiation is determined on the basis of the pulse width "b" of Fresnel reflection light detected by a light receiving section and is set slightly greater than this pulse width "b". The differentiated values of the acquired data are constant where there is no fiber event. A change in the differentiated values is used as a reference point X1 for the event and the value "a" is added to the X1 value to determine the location of the event.
Currently, prior art methods for locating non-reflective events are approximations of the event location. Accurately locating a non-reflective event depends on the amount of noise in the acquired data and the sample spacing of the data. At the present time, the most accurate non-reflective event location is no better than plus or minus one-half the sample spacing. Adding noise to the acquired data decreases the accuracy even further. What is needed is a method for locating a non-reflective event in acquired data from a fiber under test that is substantially more accurate than the present two point method or differentiating the acquired data. Such a method should not require additional sampling to improve results by reduce noise nor should it require reduced sample spacing. Such a method should be able to locate a non-reflective event to an accuracy greater than the sample spacing of the acquired data.